Issue add_field_value should use Resource update function
Issue add_field_value should use Resource update function

If add_field_value uses self.update, json data {'field': {'update': ...}} is generated, which is not accepted by the JIRA REST api.
awurster/jira,VikingDen/jira,milo-minderbinder/jira,dwmarshall/pycontribs-jira,rayyen/jira,kinow/jira,m42e/jira,akosiaris/jira,kinow/jira,m42e/jira,milo-minderbinder/jira,stevencarey/jira,coddingtonbear/jira,akosiaris/jira,rayyen/jira,dbaxa/jira,dbaxa/jira,dwmarshall/pycontribs-jira,awurster/jira,pycontribs/jira,stevencarey/jira,coddingtonbear/jira,VikingDen/jira,pycontribs/jira